Developments in semiconductor processing in recent years have steadily decreased the size of features or elements in integrated circuits (ICs). The continuous scaling of very large scale integrated circuit technologies has required dimensions of gates in field effect transistors in integrated circuits and devices to shrink to 45 to 50 nanometers (nm) in order to provide the desired performance.
In memory devices, such as static random access memory (SRAM) and dynamic random access memory (DRAM) technologies, the gate is typically formed from a complex stack of materials including polycrystalline silicon (poly), a metallic layer and an insulating cap. In the latest generation of devices, the process of forming the gates includes a selective oxidation (SELOX) process to selectively oxidize silicon without oxidizing metallic layers, in order to repair damage near the boundary of the gate stack and the gate oxide layer. SELOX can cause formation of whisker and/or bump type defects on the sidewalls of the gate structure, particularly those structures containing tungsten (W) and titanium (Ti). Examples of these defects are shown in FIGS. 1-4.
FIGS. 1-4 are SEM and TEM images of whiskers defects after SELOX on a W-containing gate structure. FIG. 1 is a planar view of a gate structure following SELOX and showing a whisker defect. FIG. 2 is a perspective view of another gate structure following SELOX and showing a whisker. FIG. 3 is a sectional side view of the gate structure of FIG. 1 showing the whisker, and FIG. 4 is a sectional side view of the gate structure and whisker of FIG. 2. These whiskers, which can be as long as the gate to gate spacing, and bump defects, which can be up to 6 nm, remain on the gate and can originate shorts in self-aligned contact (SAC) structures, increasing leakage current in SAC structures, drastically reducing reliability of the devices and leading to failure of the ICs.
Methods of preventing these failures have focused on improving and optimizing the SELOX process to prevent formation of whisker and bump defects. Although this approach has significantly reduced the occurrence of these defects, it has not been wholly successful.